stateMonad

continuePassingStyle.js

const fib_cps = (n, k) => {
  console.log(n);
  if (n < 2) {
    return k(1);
  }
  else {
    return fib_cps(n - 1, c1 => {
      return fib_cps(n - 2, c2 => {
        return k(c1 + c2);
      })
    });
  }
}

const ret_cps = fib_cps(6, x => x);
console.log('The final result is ' + ret_cps);

cpsExample.js

var sumOfNullableList = (ls, k) => {
  if(ls.length){
    if(ls[0] == null) {
      return -1;
    }
    else {
      return sumOfNullableList(ls.splice(1), v => k(v+ls[0]));
    }
  }
  else {
    return k(0); 
  }
}

console.log(sumOfNullableList([1, 2, 3, 4], x=>x));
console.log(sumOfNullableList([1, 2, null, 4], x=>x));

const factUntil = (n, limit, k) => {
  if (n < 2) {
    return k(1);
  }
  if (k(n) > limit) {
    return n;
  }
  else {
    return factUntil(n - 1, limit, (v) => k(n * v));
  }
}

console.log(factUntil(5, 10, x => x));
console.log(factUntil(5, 200, x => x));

stateMonad.js

const injectState = x => s => [x, s];

const bindState = (ma, f) => s => {
  const vs = ma(s);
  return f(vs[0])(vs[1]);
};

const getState = s => [s, s];

const putState = newS => s => [null, newS];

const fib = n => {
  console.log('fib', n);
  return n < 2 ? 1 : fib(n - 1) + fib(n - 2);
};

const fibMonad = n => {
  console.log('fibMonad', n);
  if (n < 2) {
    return bindState(getState, state => {
      const newState = (state[n] = 1); // pretend this is pure
      return bindState(putState(newState), _ => injectState(1));
    });
  } else {
    return bindState(getState, state =>
      bindState(
        state[n - 1] ? injectState(state[n - 1]) : fibMonad(n - 1),
        fib_1 =>
          bindState(
            state[n - 2] ? injectState(state[n - 2]) : fibMonad(n - 2),
            fib_2 => {
              const result = fib_1 + fib_2;
              const newState = (state[n] = result); // pretend this is pure
              return bindState(putState(newState), _ => injectState(result));
            }
          )
      )
    );
  }
};

fib(6); // 13
// fib(20); // 10946

console.log(fibMonad(6)({})); // 13
// console.log(fibMonad(20)({})); // 10946

storePassingStyle.js

const fib_sps = (n, store) => {
  console.log(n, JSON.stringify(store));
  if (store[n]) {
    return [store[n], store];
  }
  else if (n < 2) {
    return [1, store];
  }
  else {
    const fib_n1 = fib_sps(n - 1, store);
    const fib_n2 = fib_sps(n - 2, fib_n1[1]);
    const ret = fib_n1[0] + fib_n2[0];
    const newStore = fib_n2[1];
    newStore[n] = ret;
    return [ret, newStore];
  }
}

const ret_sps = fib_sps(6, {});
console.log(ret_sps[0]);